1. Field of the Invention
The invention relates to a fuel cell stack structure and, more particularly, to a solid-polyelectrolyte fuel cell (PEMFC) stack structure.
2. Description of the Related Art
A solid-polyelectrolyte fuel cell is constructed by laminating membrane-electrode assemblies (MEA's) and separators. Each of the MEA's is composed of an electrode membrane made of an ion exchange membrane, an electrode (anode or fuel pole) made of a catalytic layer disposed on one face of the electrode membrane, and an electrode (cathode or air pole) made of a catalytic layer disposed on the other face of the electrode membrane. Each of the separators has a fluid passage for supplying the anode and cathode of a corresponding one of the MEA's with fuel gas (hydrogen) and oxidative gas (oxygen, usually air) respectively. Each of the separators also has a coolant flow channel through which coolant flows. A diffusion layer is interposed between each of the MEA's and a corresponding one of the separators. One or more cells are laminated to constitute a module. The same modules as this one are laminated to constitute a module group. A terminal, an insulator, and an end plate are disposed on either side of the module group in a directions in which the cells are laminated (hereinafter referred to as a cell-lamination direction). A laminated-cell body thus constructed is fastened in the cell-lamination direction. The laminated-cell body is fixed on its outside by a fastening member (e.g., a tension plate, a tension bolt, or the like), whereby a stack is constructed.
On the anode side of the solid-polyelectrolyte fuel cell, a reaction of turning one hydrogen molecule into two hydrogen ions and two electrons occurs, and the hydrogen ions move through an electrolytic membrane toward the cathode side. On the cathode side of the solid-polyelectrolyte fuel cell, a reaction of producing two water molecules from four hydrogen ions, four electrons, and one oxygen molecule (the electrons produced in the anode of an adjacent one of the MEA's penetrate a corresponding one of the separators, or the electrons produced in the anode of a cell on one end of the laminated-cell body flow to the cathode of a cell on the other end of the laminated-cell body through an external circuit) occurs.
Anode Side: H2→2H++2e−
Cathode Side: 2H++2e−+(½)O2→H2O
In order for hydrogen ions to move through the electrolytic membrane, it is required that the electrolytic membrane be suitably wet. In addition to humidifying gas appropriately and supplying it to the laminated-cell body, water produced by power-generating reactions mentioned above is utilized to keep the electrolytic membrane wet. However, if the electrolytic membrane becomes excessively wet, water pockets (flooding) are created in gas flow channels. This causes a decrease in output of the fuel cell.
According to a procedure proposed in Japanese Patent Application Laid-Open No. 2001-236975, a bypass flow channel for gases irrelevant to power generation is formed in a deep end portion of a fuel cell stack, produced water that has flown through this bypass flow channel to stay in a gas manifold on the gas outlet/inlet side is extruded, and the occurrence of inconveniences resulting from the produced water is restrained.
However, the fuel cell stack of the related art has a problem, namely, a drop in the voltage of end cells during power generation. This problem is caused mainly because of the following reasons. The first one (1) consists in that condensate and impurities (metal ions contained in a system and the like) tend to mix with the end cells, that flooding or contamination is thus caused, and that cell voltages are decreased as a result. The second one (2) consists in that the end portions are susceptible to external heat and thus tend to be cooled, and that flooding occurs as a result.
Even if the bypass flow channel for gases is formed in the deep end at the outlet or inlet of the stack as disclosed in Japanese Patent Application Laid-Open No. 2001-236975, a drop in voltage in those cells at the gas outlet/inlet ends is inevitable. Especially in the cells at the gas outlet/inlet ends, inconveniences are likely to be caused by impurities that have mixed with gas. These inconveniences cannot be eliminated by the aforementioned related art. In addition, there is caused another problem, which is the occurrence of flooding based on the fact that the cells at the gas outlet/inlet ends tend to be cooled. This problem cannot be solved either.
It is an object of the invention to provide a fuel cell stack structure capable of suppressing a drop in voltage resulting from flooding or contamination in end portions in the cell-lamination direction, particularly, in the cells at the gas outlet/inlet ends.